KR20230154955A - Steel plates and hot stamping members for hot stamping - Google Patents

Abstract

A steel sheet for hot stamping having a portion on the surface having a surface-treated film having an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. and a portion not having the surface-treated film, wherein the surface-treated film includes carbon black, Contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and the carbon black and the oxide are dispersed throughout the surface treatment film, and the surface treatment film When the content of silver and silica is 0 to 0.3 g/m2, and the adhesion amounts of the carbon black and the oxide are respectively X _CB (g/m2) and X _Oxide (g/m2), the following formula is satisfied. 118.9≤24280/{6700/(100+76×X _CB )+18000/(130+65×X _Oxide )}≤332.0

Description

Steel plates and hot stamping members for hot stamping

The present invention relates to a steel sheet for hot stamping and a hot stamping member.

In recent years, in order to protect the environment and prevent global warming, there have been increasing calls to curb the consumption of chemical fuels, and this request is having an impact on various manufacturing industries. For example, automobiles, which are essential for daily life and activities as a means of transportation, are no exception, and improvements in fuel efficiency by reducing the weight of the vehicle body, etc. are required. However, in automobiles, simply reducing the weight of the car body may lead to a decrease in safety, so it is not acceptable in terms of product quality. Therefore, when reducing the weight of a vehicle body, it is necessary to ensure appropriate safety.

Most of the structure of an automobile is made of iron, especially steel plates, and reducing the weight of the steel plates is important for reducing the weight of the vehicle body. In addition, requests for such steel plates are being made not only in the automobile manufacturing industry but also in various manufacturing industries. In response to such a request, if the goal is simply to reduce the weight of the steel sheet, it may be considered to reduce the thickness of the steel sheet. However, thinning the thickness of the steel plate leads to a decrease in the strength of the structure. Therefore, in recent years, research and development has been conducted on steel sheets that can maintain or increase the mechanical strength of structures made of steel sheets by increasing the mechanical strength of steel sheets, even if they are thinner than previously used steel sheets.

In general, materials with high mechanical strength tend to have lower shape freezing properties during molding processing such as bending processing. Therefore, when processing into a complex shape, the processing itself becomes difficult. As one of the means of solving this problem regarding formability, the so-called "hot stamping method" (also called the hot pressing method, hot pressing method, high temperature pressing method, and die stamping method) can be mentioned. In this hot stamping method, the material to be molded is heated to a high temperature to transform (austenitize) into a structure called austenite, and the steel sheet softened by heating is pressed and molded, and then cooled after molding. According to this hot stamping method, the material is first heated to a high temperature to soften it, so the material can be easily pressed. Additionally, the mechanical strength of the material can be increased due to the quenching effect caused by cooling after molding. Therefore, by this hot stamping method, a molded article with good shape freezing properties and high mechanical strength can be obtained.

On the other hand, depending on the part, there are cases where it is required to have both a high-strength part and a low-strength part. This is because, while high strength is required to protect personnel, relatively low strength is needed to absorb energy during a collision in areas other than those where personnel are to be protected.

From the above viewpoint, for example, Patent Document 1 below describes a part obtained by joining at least two types of Zn-based plated steel sheets with different steel components. Additionally, Patent Document 2 below describes a technique for separately producing the structure after cooling by setting differences in the average cooling rate and cooling stop temperature depending on the location of the steel sheet. Additionally, Patent Document 3 below describes a technique for increasing ductility by controlling the temperature of the mold and tempering a portion of the mold after martensite transformation.

International Publication No. 2016/139953 Japanese Patent Publication No. 2014-161854 Japanese Patent Publication No. 2016-41440

However, in order to manufacture parts as described in Patent Document 1, it is required to manufacture steel plates with different strengths, so in addition to increasing the manufacturing load, a joining process is required, which is not economical. .

Additionally, using the method described in Patent Document 2 is not preferable because it requires two types of molds for cooling, which is economically disadvantageous and makes it difficult to secure robustness in manufacturing.

In addition, even when the technology described in Patent Document 3 is used, it is not preferable because it becomes difficult to ensure robustness of production as in Patent Document 2.

In this way, there is a current need for a technology that can perform hot stamping while suppressing the increase in cost and ensuring the robustness of the product.

Therefore, the present invention was made in view of the above problems, and the object of the present invention is to provide a steel sheet for hot stamping suitable for manufacturing parts having parts with different strengths by hot stamping, and parts with different strengths. The object is to provide a hot stamp member having.

In order to solve the above problem and manufacture metal parts having parts with different strengths, the present inventors conducted intensive studies and found that by applying a high emissivity surface treatment film to a part of the surface of the steel plate, the temperature increase rate (temperature increase rate) The idea was to increase .

By applying a surface treatment film that increases the emissivity to a part of the steel sheet, it is possible to set a difference in the temperature increase rate during hot stamping heating. A steel sheet for hot stamping having such characteristics is taken out from the heating furnace with the portion to which the film has been applied having completed austenitization and the other portions with uncompleted austenitization, and is rapidly cooled by a mold. As a result, a difference in hardening properties occurs between the portion to which the film is applied and the other portion, making it possible to manufacture parts derived from steel sheets having portions with different strengths.

The present invention has been made based on the above findings obtained by the present inventors, and the gist of the present invention is as follows.

[1] A steel sheet has a portion on the surface of a steel sheet having a surface treatment film having an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. and a region without the surface treatment film, the surface treatment film comprising carbon black, Contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and the carbon black and the oxide are dispersed throughout the surface treatment film, and the surface treatment film When the content of silver and silica is 0 to 0.3 g/m2, and the adhesion amounts of the carbon black and the oxide are respectively X _CB (g/m2) and X _Oxide (g/m2), the following equation (1) is satisfied. Shiki, steel plate for hot stamping.

118.9≤24280/{6700/(100+76×X _CB )+18000/(130+65×X _Oxide )}≤332.0 … Equation (1)

[2] The steel sheet for hot stamping according to [1], wherein the surface treatment film contains 5.0 to 40.0 volume% of the carbon black and 1.0 to 30.0 volume% of the oxide.

[3] The ratio _of the adhesion amount of the carbon black X _CB (g/m2) and the adhesion amount of the _{oxide X Oxide (} g/m2) Steel plate for hot stamping.

[4] The steel sheet for hot stamping according to any one of [1] to [3], wherein _the adhesion _amount of carbon black

[5] The steel sheet for hot stamping according to any one of [1] to [4], wherein the surface treatment film has an emissivity of 60% or more at a wavelength of 8.0 μm at 700°C.

[6] The steel sheet for hot stamping according to any one of [1] to [5], wherein the steel sheet for hot stamping has a metal plating layer on one or both sides of the steel sheet between the base material of the steel sheet and the surface treatment film.

[7] On the surface of the steel sheet, there is a portion having a surface treatment film and a portion not having the surface treatment film, and the surface treatment film is one selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. A hot stamp member containing more than one type of _oxide , the adhesion amount of the oxide

[8] When measuring the Vickers hardness specified in JIS Z 2244 (2009), there is a part showing the maximum hardness HVmax and a part showing the minimum hardness HVmin, and the hardness of the maximum hardness HVmax and the minimum hardness HVmin is present. The hot stamp member described in [7], wherein the difference ΔHV is 150 or more.

[9] The hot stamp member according to [8], wherein both the portion showing the maximum hardness HVmax and the portion showing the minimum hardness HVmin exist in a region composed of a common material.

As described above, according to the present invention, it is possible to provide a steel sheet for hot stamping suitable for manufacturing parts having parts with different strengths and a hot stamping member having parts with different strengths.

Figure 1 is a diagram schematically showing the start time and completion time of austenitization in the coated portion and the non-coated portion during hot stamp heating.
FIG. 2 is a diagram schematically showing the oil-coated portion and its central portion (P1), and the non-coated portion and its central portion (P2) on the surface of the steel sheet for hot stamping.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

(Steel plate for hot stamping)

The steel sheet for hot stamping according to the embodiment of the present invention described below is suitable for manufacturing parts having parts with different strengths. Such a steel sheet for hot stamping has a portion on the surface of the steel sheet that has a surface treatment film with an emissivity of 60% or more at a wavelength of 8.0 μm at 25°C and a region that does not have this surface treatment film. In this embodiment, by applying a surface treatment film to a part of the steel sheet as described above, the emissivity at a wavelength of 8.0 μm at 25° C. can be increased to 60% or more.

In the steel sheet for hot stamping according to this embodiment, the type of steel sheet (base steel sheet) serving as the base material is not particularly limited. Examples of such steel sheets include various hot-rolled steel sheets, cold-rolled steel sheets, and plated steel sheets. Coated steel sheets include, for example, steel sheets that have been subjected to hot-dip aluminum plating, hot-dip zinc plating, alloyed hot-dip zinc plating, and electrogalvanization. However, as long as they can be applied to hot stamping, they are not limited to these plated steel sheets.

Conventionally, most of the steel sheets used as automobile frame parts, etc. were hot-rolled steel sheets, cold-rolled steel sheets, or plated steel sheets plated with aluminum, zinc, etc. Since these steel sheets have a low emissivity, the temperature increase rate for radiative heating around a wavelength of 8.0 μm is low.

In this embodiment, as schematically shown in FIG. 1, a specific surface treatment film is applied to only a part of the surface of the steel sheet, thereby increasing the heating rate of the oil film portion during hot stamp heating. After the oil-coated portion reaches a temperature above the Ac3 point and the non-coated portion reaches a temperature below the Ac3 point, the steel sheet is hot stamped to locally transform the oil-coated portion into a martensite structure. makes it possible.

In this embodiment, a surface treatment film with a high emissivity is applied to a part of the surface of the steel sheet. Specific methods for applying a surface treatment film include methods such as painting and laminating, but are not limited to these methods. The surface treatment film as described above may be applied to only one side of the steel sheet or to both sides of the steel sheet. The emissivity of the area on the surface of the steel sheet to which the surface treatment film has been applied is 60% or more at a wavelength of 8.0 μm at 25°C. The emissivity at a wavelength of 8.0 μm at 25°C in the area where the surface treatment film is applied is preferably 70% or more, and more preferably 80% or more.

Additionally, the emissivity measurement method can be performed as described in JIS R 1801 (2002) of the Japanese Industrial Standard. In this case, the sample is set in a Fourier transform infrared spectrophotometer, the radiation intensity at a wavelength of 8.0 μm is measured at 25°C, and the emissivity is calculated.

Additionally, it is also possible to measure the radiation intensity of the area of interest at 25°C using a radiation thermometer with the measurement wavelength set to 8.0 μm and calculate the emissivity from the ratio to the radiation intensity of the black body.

When applying a surface treatment film to only a part of the surface of a steel sheet by painting, for example, an organic or inorganic treatment liquid containing carbon black and metal oxide is applied to a part of the surface of the steel sheet using a roll coater or curtain coater. A surface treatment film can be applied by drying the volatile components in the treatment liquid. Additionally, a film can be applied to an arbitrary portion of the surface of the steel sheet by covering a part of the steel sheet with polyester tape or another material and then subjecting it to painting using the treatment liquid.

In particular, in inkjet coating, the treatment liquid can be applied with high precision to an arbitrary position, and it is also possible to continuously change the film thickness.

<Surface treatment film>

In the steel sheet for hot stamping according to the present embodiment, the emissivity of the portion to which the surface treatment film has been applied is 60% or more at a wavelength of 8.0 μm at 25°C. When the emissivity at a wavelength of 8.0 μm at 25°C is less than 60%, the difference in temperature increase rate from the portion where the surface treatment film is not applied becomes small. As a result, the difference in hardness after heating and cooling in the hot stamp cannot be secured, or the degree of freedom in the timing of taking out the steel sheet from the heating furnace to ensure it is lowered, which places a burden on manufacturing, making it possible to achieve both weight reduction and safety performance as an automobile part. This becomes difficult. The emissivity at a wavelength of 8.0 μm at 25°C is preferably 80% or more. When the emissivity at a wavelength of 8.0 μm at 25°C is 80% or more, it becomes easier to secure the hardness difference and it becomes possible to further reduce the load on manufacturing. In addition, the higher the emissivity at a wavelength of 8.0 μm at 25°C of the portion to which the surface treatment film has been applied, the higher the better, and the upper limit is not specified, and may be 100%.

In order to achieve an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C., the surface treatment film according to the present embodiment contains carbon black and a specific metal oxide as described in detail below. In addition, the surface treatment film according to the present embodiment may further contain a binder component, various additives, etc. as needed. In addition, the surface treatment film according to this embodiment does not have to contain silica, and may contain silica within a certain range. By adjusting the content of carbon black and metal oxide, the thickness of the surface treatment film, etc., it becomes possible to achieve the desired emissivity.

More specifically, the surface treatment film according to the present embodiment contains carbon black and at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and also includes carbon black and the above oxide. Silver exists dispersed throughout the surface treatment film. In addition, _when the adhesion amount of carbon black is expressed as X _CB (g/m2) and the adhesion amount of oxide is expressed as Satisfying relationships.

118.9≤24280/{6700/(100+76×X _CB )+18000/(130+65×X _Oxide )}≤332.0 … Equation (1)

The above (Equation 1) defines the relationship between the increase rate (%) of the temperature increase rate (°C/s) and the adhesion amount of carbon black and oxide. More specifically, regarding the increase in temperature increase rate, carbon black functions as a heat absorber in the range up to 700°C, and in the range above 700°C, Zr oxide, Zn oxide, and Ti oxide remaining even in this temperature range. It is formalized to function as a heat absorber.

As briefly mentioned above, the surface treatment film according to the present embodiment can be formed by applying a treatment liquid containing carbon black and a specific oxide to a desired portion of the steel sheet. As a result, in the surface treatment film according to the present embodiment, carbon black and oxide exist dispersed throughout the surface treatment film. By dispersing the carbon black and the oxide throughout the surface treatment film, the emissivity of the surface treatment film at a wavelength of 8.0 μm at 25°C can be made uniform throughout the film. As a result, when the area on which the surface treatment film according to the present embodiment has been applied is hot stamped, the entire surface treatment film can be heated without unevenness.

The distribution state of such carbon black and oxide can be determined by measuring the surface treatment film using an Electron Probe Micro Analyzer (EPMA) to determine whether elements derived from carbon black (e.g., C) or oxides are derived. This can be confirmed by surface analysis of the elements (i.e. Zr, Zn, Ti).

Additionally, when a treatment liquid containing carbon black and a treatment liquid containing an oxide are prepared separately and each of these treatment liquids is applied to form a laminated film, the carbon black and the oxide are dispersed throughout the film. It does not come into existence. In addition, when attempting to form a film using a plurality of treatment liquids in this way, the second layer film must be formed after the first layer film is formed, so the manufacturing equipment becomes larger and the manufacturing cost also increases.

In addition, the surface treatment film according to the present embodiment has an emissivity of 60 at a wavelength of 8.0 μm at 25°C by satisfying the relationship expressed by the above equation (1) where the adhesion _amount of carbon _black % or more, the difference in temperature increase rate from the area where the surface treatment film does not exist becomes significant. If the value specified in the central term of the above equation (1) is less than 118.9, the amount of carbon black and oxide attached is insufficient, and the above emissivity cannot be achieved. The value specified in the central term of the above equation (1) is preferably 119.0 or more, more preferably 170.0 or more, and even more preferably 220.0 or more. On the other hand, if the value specified in the central term of the above equation (1) exceeds 332.0, it is not preferable because the adhesion of the film decreases. The value specified in the central term of the above formula (1) is preferably 330.0 or less, more preferably 310.0 or less, and even more preferably 300.0 or less.

Here, the adhesion amount of carbon _black That is, a cross-sectional analysis is performed using TEM-EDS analysis for the area indicated by the film thickness x 5㎛, and the film thickness of the surface-treated film and the area ratio occupied by particles with a carbon content of 70% by mass or more are measured. . When the film thickness is d (μm) and the area ratio is a (%), the value expressed as d × a becomes the adhesion _amount of carbon black

In addition _{, the} adhesion _amount , metal Zn, and metal Ti, meaning the amount attached per unit area. The adhesion amount of these _oxides

The steel sheet for hot stamping according to the present embodiment has the above-described characteristics, making it possible to set the emissivity at 700°C and a wavelength of 8.0 μm to 60% or more. Hereinafter, the materials contained in the surface treatment film, which are characteristic for realizing the above emissivity, will be explained in more detail.

≪Carbon black≫

In the area of the surface of the steel sheet to which the surface treatment film has been applied, the adhesion amount X _CB of the carbon black in the surface treatment film is preferably 0.030 g/m 2 or more. By setting the adhesion _amount The adhesion amount X _CB is more preferably 0.100 g/m 2 or more. On the other hand, the upper limit of the adhesion _amount The _adhesion amount

Moreover, it is more preferable that the surface treatment film contains 5.0 to 40.0% by volume of carbon black. Carbon black is particularly effective in increasing the temperature increase rate in the range up to 700°C. The thickness of the surface treatment film may vary locally depending on the roughness or undulations of the steel sheet, or differences in the rate of volatilization of volatile components such as water in the treatment liquid during film formation. At this time, when the carbon black content is 5.0% by volume or more, the difference between the parts that appear black due to the carbon black and the other parts can be suppressed, and designability can be maintained, which is preferable in terms of appearance. On the other hand, when the carbon black content in the surface treatment film is 40.0% by volume or less, a decrease in paint adhesion after hot stamping can be suppressed. Although the mechanism is not clear, it is presumed that inhibition of the bond between the paint and the base material is suppressed by suppressing the remaining carbon black or compounds derived from carbon black oxides, etc.

Additionally, since the main components of carbon black are carbon, hydrogen, and oxygen, carbon black disappears when heated to high temperatures. Therefore, by containing carbon black in the surface treatment film, the performance after hot stamping, such as adhesion after painting, can be maintained.

When the carbon black content is 5.0% by volume or more, oxidation of the steel sheet or plating layer located below the surface treatment film is suppressed, and reactivity when the paint (treatment agent) is applied is ensured, enabling a strong bond. Paint adhesion is maintained. Additionally, the emissivity can be increased, and the temperature increase rate can be increased. It is more preferable that the carbon black content in the surface treatment film is 8.0 volume% or more. By setting the carbon black content in the surface treatment film to 8.0% by volume or more, it becomes possible to further increase the temperature increase rate. On the other hand, when the carbon black content in the surface treatment film is 40.0% by volume or less, the effect of increasing the emissivity can be sufficiently obtained while suppressing an increase in film cost. It is more preferable that the carbon black content in the surface treatment film is 30.0 volume% or less. By setting the carbon black content in the surface treatment film to 30.0% by volume or less, it becomes possible to further suppress film costs.

≪Metal oxide≫

On the surface of the steel sheet, in the area where the surface treatment film is applied, the total adhesion amount of Zr oxide, Zn oxide, and Ti _oxide in the surface treatment film By setting the _adhesion amount The _adhesion amount On the other hand, the upper limit of the adhesion _amount The _adhesion amount

Moreover, it is more preferable that the surface treatment film contains Zr oxide, Zn oxide, and Ti oxide in a total of 1.0 to 30.0% by volume. Oxides of these elements (i.e., metal oxides of Zr, Zn, and Ti) remain in the surface treatment film even when heated to a high temperature of 700°C or higher, which reduces the effect of carbon black. As a result, in the area where the surface treatment film is applied, the amount of heat input due to radiant heat from the heating atmosphere increases because these metal oxides have a higher emissivity than the surface of the steel sheet or the plating surface at a high temperature of 700 ° C. or higher. This makes it possible to maintain the effect of increasing the temperature increase rate even at high temperatures of 700°C or higher. When the content of these metal oxides is 1.0% by volume or more, the effect of increasing the temperature increase rate can be sufficiently obtained. It is more preferable that the content of these metal oxides is 3.0 volume% or more. On the other hand, when the content of these metal oxides is 30 volume% or less, the film cost can be suppressed, which is economically preferable. It is more preferable that the content of these metal oxides is 25.0 volume% or less.

In addition, the content (volume %) of various compounds such as carbon black and metal oxide in the surface treatment film was determined by embedding the sample in resin, polishing, and observing the cross section with a scanning electron microscope (SEM). It can be calculated from the area ratio. Additionally, compounds can be estimated by quantitative analysis using the EDX function attached to SEM.

In the surface _{treatment film} according to the present _embodiment , the ratio of the adhesion amount of carbon _black . When _{the ratio The ratio}

Additionally, the surface treatment film according to the present embodiment can contain various binder components and additives in addition to the carbon black and metal oxide.

≪Binder ingredient≫

The binder component that may be contained in the surface treatment film according to the present embodiment is preferably a water-dispersible or water-soluble resin. The content of the binder component selected from water-dispersible or water-soluble resin is preferably 40% by volume or more with respect to the total volume of the surface treatment film.

As the binder component selected from water-dispersible or water-soluble resins, it is possible to use various known resins that exhibit water-dispersible or water-soluble resins. Resins that exhibit such water dispersibility or water solubility include, for example, polyurethane resins, polyester resins, acrylic resins, epoxy resins, fluorine resins, polyamide resins, polyolefin resins, and polymers obtained by hydrolysis and condensation polymerization of silane coupling agents. Compounds, etc. can be mentioned. This binder component is more preferably one or two or more selected from the group consisting of polyester resin, polyurethane resin, polyolefin resin, acrylic resin, epoxy resin, fluorine resin, and polyamide resin. In addition, when using a plurality of resins as a binder component, the total content of the plurality of resins used is treated as the content of the binder component.

In addition, when using a polyurethane resin as a binder component, it is preferable that the polyurethane resin is a polyether-based polyurethane resin. This is because, by using a polyether-based polyurethane resin, hydrolysis due to acids or alkalis can be prevented compared to polyester-based polyurethane resin, and compared to polycarbonate-based polyurethane resin. This is because, by suppressing the formation of a hard and soft film, adhesion during processing and corrosion resistance of the processed area can be ensured.

Whether polyurethane resin is contained is determined by the infrared absorption spectrum obtained by infrared spectroscopy: 3330 cm ^-1 (NH stretching), 1730 cm ^-1 (C=O stretching), 1530 cm ^-1 (CN), 1250 cm ^-1 ( A judgment can be made based on whether characteristic absorption of CO) is observed. Also, regarding the content of the polyurethane resin, the content can be specified from the intensity of the obtained characteristic absorption by using a sample whose content is known in advance and creating a calibration curve showing the relationship between the content and the intensity of characteristic absorption.

In addition, with respect to resins other than the above polyurethane resins, it is possible to determine the presence or absence of content and content, as with the above polyurethane resins, by paying attention to the characteristic absorption derived from the functional group unique to each resin.

≪Additives≫

The surface treatment film according to the present embodiment contains various additives such as leveling agents, water-soluble solvents, metal stabilizers, etching inhibitors, etc. as additives in preparing the treatment solution before forming the film, to the extent that the effect of the present invention is not impaired. It is possible to do so.

Examples of the leveling agent include nonionic or cationic surfactants, such as polyethylene oxide or polypropylene oxide adducts, and acetylene glycol compounds.

Examples of water-soluble solvents include alcohols such as ethanol, isopropyl alcohol, t-butyl alcohol and propylene glycol, cellosolves such as ethylene glycol monobutyl ether and ethylene glycol monoethyl ether, and esters such as ethyl acetate and butyl acetate. ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone.

Examples of the metal stabilizer include chelate compounds such as EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriamineoacetic acid).

Examples of the etching inhibitor include amine compounds such as ethylenediamine, triethylenepentamine, guanidine, and pyrimidine.

Additionally, the content of the binder component or additive can be measured in the same manner as in the case of carbon black or metal oxide.

≪Silica≫

As mentioned above, the surface treatment film according to the present embodiment does not have to contain silica, and may contain silica within a certain range. More specifically, in the surface treatment film according to the present embodiment, the silica content is 0 to 0.3 g/m2. When silica is contained in excess of 0.3 g/m2, it is difficult to expect a temperature increase effect and the cost is high, so it is undesirable from the viewpoint of economic efficiency. Additionally, since silica is a material with low electrical conductivity, when silica is contained in excess of 0.3 g/m2, it is undesirable in terms of weldability after hot stamping. In the case of containing silica, the smaller the silica content of the surface treatment film, the better. The silica content of the surface treatment film is more preferably 0.10 g/m 2 or less, and even more preferably 0.05 g/m 2 or less.

≪Film thickness of surface treatment film≫

The film thickness of the surface treatment film containing the above components is preferably, for example, 0.5 to 5.0 μm. By keeping the film thickness of the surface treatment film within the above range, the emissivity at a wavelength of 8.0 μm at 25°C can be more reliably set to 60% or more. The film thickness of the surface treatment film is more preferably 1.0 to 3.0 μm.

<Metal plating layer>

The hot stamping steel sheet according to the present embodiment preferably has a metal plating layer on one or both sides of the hot stamping steel sheet on at least a portion between the base steel sheet and the surface treatment film. By having a metal plating layer, corrosion resistance after hot stamping and painting can be further improved. Additionally, the presence of a metal plating layer can prevent iron scale from being generated by heating during hot stamping. Iron scale contaminates the heating furnace or adheres to rolls used for conveyance, thereby becoming a manufacturing burden. Therefore, when iron scale is generated, a process such as shot blasting is required to remove the iron scale, which is economically undesirable.

The type of metal plating layer is not particularly limited. Examples of the metal plating constituting this metal plating layer include aluminum plating, Al-Si plating, zinc plating, alloyed zinc plating, Zn-Ni plating, Zn-Al-Mg plating, and Zn-Al-Mg-Si plating. there is.

In addition, methods for forming the metal plating layer include, but are not particularly limited to, hot dip plating, electroplating, physical vapor deposition, and chemical vapor deposition.

<Base steel plate>

Next, the base steel sheet of the hot stamping steel sheet according to the present embodiment is not particularly limited as long as it is a steel sheet that can be suitably used in the hot stamping method. A steel sheet applicable to the hot stamping steel sheet according to the present embodiment, for example, the chemical composition in mass% is C: 0.10 to 0.40%, Si: 0.01 to 0.60%, Mn: 0.50 to 3.00%, and P: 0.05%. Hereinafter, a steel sheet containing S: 0.020% or less, Al: 0.10% or less, Ti: 0.01 to 0.10%, B: 0.0001 to 0.0100%, and N: 0.010% or less, with the balance consisting of Fe and impurities, can be exemplified. . In addition, examples of the form of the base steel sheet include steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets. Hereinafter, the chemical composition of the base steel sheet will be described in detail. In addition, in the following description of the chemical composition of the base steel sheet, the notation of “%” means “% by mass” unless otherwise specified.

[C: 0.10 to 0.40%]

C is contained to ensure the desired mechanical strength. When the C content is 0.10% or more, sufficient improvement in mechanical strength is obtained, and the effect of containing C is sufficiently obtained. Therefore, it is preferable that the C content is 0.10% or more. The C content is more preferably 0.20% or more. On the other hand, when the C content is 0.40% or less, the strength of the steel sheet can be improved through hardening and the decrease in elongation and drawing can be suppressed. Therefore, it is preferable that the C content is 0.40% or less. The C content is more preferably 0.35% or less.

[Si: 0.01 to 0.60%]

Si is one of the strength improving elements that improves mechanical strength, and like C, it is contained to ensure the desired mechanical strength. When the Si content is 0.01% or more, the strength improvement effect is sufficiently exhibited, and sufficient improvement in mechanical strength is obtained. Therefore, it is preferable that the Si content is 0.01% or more. The Si content is more preferably 0.10% or more. On the other hand, since Si is also a reverse oxidation element, when the Si content is 0.60% or less, the decline in wettability during hot-dip Al plating due to the influence of Si oxide formed on the surface layer of the steel sheet is suppressed, preventing the occurrence of non-plating. It can be suppressed. Therefore, it is preferable that the Si content is 0.60% or less. The Si content is more preferably 0.40% or less.

[Mn: 0.50 to 3.00%]

Mn is one of the reinforcing elements that strengthens steel, and is also one of the elements that improves hardness. Additionally, Mn is an element effective in preventing hot embrittlement caused by S, which is one of the impurities. When the Mn content is 0.50% or more, these effects are sufficiently obtained. Therefore, in order to reliably achieve the above effect, the Mn content is preferably 0.50% or more. The Mn content is more preferably 0.80% or more. On the other hand, since Mn is an austenite forming element, when the Mn content is 3.00% or less, the retained austenite phase does not increase too much and the decrease in strength is suppressed. Therefore, it is preferable that the Mn content is 3.00% or less. The Mn content is more preferably 1.50% or less.

[P: 0.05% or less]

P is an impurity contained in steel. When the P content is 0.05% or less, P contained in the steel sheet can be suppressed from segregating at the grain boundaries of the steel sheet and lowering the base material toughness of the hot stamped molded body, thereby suppressing a decrease in the delayed fracture resistance of the steel sheet. Therefore, the P content is preferably 0.05% or less, and it is desirable to keep the P content as small as possible.

[S: 0.020% or less]

S is an impurity contained in steel. When the S content is 0.020% or less, it is possible to prevent S contained in the steel sheet from forming sulfides and lowering the toughness of the steel sheet, thereby suppressing a decrease in the delayed fracture resistance of the steel sheet. Therefore, the S content is preferably 0.020% or less, and it is desirable to keep the S content as small as possible.

[Al: 0.10% or less]

Al is generally used for deoxidation purposes in steel. On the other hand, when the Al content is 0.10% or less, the increase in the Ac3 point of the steel sheet is suppressed, so the heating temperature required to ensure hardening properties of the steel during hot stamping can be reduced, which is desirable for hot stamping manufacturing. Therefore, the Al content of the steel sheet is preferably 0.10% or less, more preferably 0.05% or less, and even more preferably 0.01% or less.

[Ti: 0.01 to 0.10%]

Ti is one of the strength enhancing elements. When the Ti content is 0.01% or more, the strength improvement effect and the oxidation resistance improvement effect are sufficiently obtained. Therefore, in order to reliably achieve the above effect, the Ti content is preferably 0.01% or more. The Ti content is more preferably 0.03% or more. On the other hand, when the Ti content is 0.10% or less, for example, the formation of carbides and nitrides is suppressed, softening of the steel can be suppressed, and the desired mechanical strength can be sufficiently obtained. Therefore, the Ti content is preferably 0.10% or less. The Ti content is more preferably 0.08% or less.

[B: 0.0001 to 0.0100%]

B has the effect of improving strength by acting upon quenching. When the B content is 0.0001% or less, this strength improvement effect is sufficiently obtained. Therefore, it is preferable that the B content is 0.0001% or more. The B content is more preferably 0.0010% or more. On the other hand, when the B content is 0.0100% or less, the formation of inclusions is reduced, embrittlement of the steel sheet is suppressed, and a decrease in fatigue strength can be suppressed. Therefore, it is preferable that the B content is 0.0100% or less. The B content is more preferably 0.0040% or less.

[N: 0.010% or less]

N is an impurity contained in steel. When the N content is 0.010% or less, the formation of nitrides due to N contained in the steel sheet can be suppressed, and a decrease in the toughness of the steel sheet can be suppressed. In addition, when B is contained in the steel sheet, N contained in the steel sheet is suppressed from combining with B to reduce the amount of dissolved B, thereby suppressing a decrease in the hardenability improvement effect of B. Therefore, the N content is preferably 0.010% or less, and it is more preferable to keep the N content as small as possible.

In addition, the base steel sheet of the hot stamping steel sheet according to the present embodiment further contains Cr, Mo, Ni, Co, Cu, Mo, V, Nb, Sn, W, Ca, REM, O, Sb as optionally added elements. It may contain elements such as

[Cr: 0 to 1.00%]

Cr is an element that improves the hardness of steel sheets. In order to sufficiently obtain this effect, it is preferable that the Cr content is 0.01% or more. On the other hand, by setting the Cr content to 1.00% or less, an increase in cost can be suppressed while sufficiently obtaining the effect. Therefore, it is preferable that the Cr content when included is 1.00% or less.

[Ni: 0 to 2.00%]

[Co: 0 to 2.00%]

Ni and Co are elements that improve the hardenability of steel and make it possible to stably secure the strength of the steel sheet member after hardening. In order to sufficiently develop this effect, the Ni content is preferably 0.10% or more, and the Co content is preferably 0.10% or more. On the other hand, when the Ni content and Co content are each 2.00% or less, the above effects are sufficiently obtained and economic efficiency increases. Therefore, the Ni content when included is preferably 2.00% or less, and the Co content is preferably 2.00% or less.

[Cu: 0 to 1.000%]

Cu is an element that improves the hardenability of steel and makes it possible to stably secure the strength of the steel sheet member after hardening. Additionally, Cu improves pitting resistance in a corrosive environment. In order to fully develop this effect, it is preferable that the Cu content is 0.100% or more. On the other hand, when the Cu content is 1.000% or less, the above effects are sufficiently obtained and economic efficiency increases. Therefore, it is preferable that the Cu content when included is 1.000% or less.

[Mo: 0 to 1.00%]

Mo is an element that improves the hardenability of steel and makes it possible to stably secure the strength of the steel sheet member after hardening. In order to sufficiently develop this effect, it is preferable that the Mo content is 0.10% or more. On the other hand, when the Mo content is 1.00% or less, the above effects are sufficiently obtained and economic efficiency increases. Therefore, it is preferable that the Mo content when included is 1.00% or less.

[V: 0 to 1.00%]

V is an element that improves the hardenability of steel and makes it possible to stably secure the strength of the steel plate member after hardening. In order to sufficiently develop this effect, it is preferable that the V content is 0.10% or more. On the other hand, when the V content is 1.00% or less, the above effects are sufficiently obtained and economic efficiency increases. Therefore, it is preferable that the V content when included is 1.00% or less.

[Nb: 0 to 1.00%]

Nb is an element that improves the quenching properties of steel and makes it possible to stably secure the strength of the steel sheet member after quenching. In order to sufficiently develop this effect, it is preferable that the Nb content is 0.01% or more. On the other hand, when the Nb content is 1.00% or less, the above effects are sufficiently obtained and economic efficiency increases. Therefore, it is preferable that the Nb content when included is 1.00% or less.

[Sn: 0 to 1.00%]

Sn is an element that improves pitting resistance in a corrosive environment. In order to fully develop this effect, it is preferable that the Sn content is 0.01% or more. On the other hand, when the Sn content is 1.00% or less, a decrease in grain boundary strength can be suppressed, and a decrease in toughness can be suppressed. Therefore, it is preferable that the Sn content when included is 1.00% or less.

[W: 0 to 1.00%]

W is an element that improves the hardenability of steel and makes it possible to stably secure the strength of the steel plate member after hardening. Additionally, W improves pitting resistance in a corrosive environment. In order to sufficiently develop this effect, it is preferable that the W content is 0.01% or more. On the other hand, when the W content is 1.00% or less, the above effects are sufficiently obtained and economic efficiency increases. Therefore, it is preferable that the W content when included is 1.00% or less.

[Ca: 0 to 0.010%]

Ca is an element that has the effect of miniaturizing inclusions in steel and improving toughness and ductility after quenching. In order to sufficiently develop this effect, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more. On the other hand, when the Ca content is 0.010% or less, the effect can be sufficiently obtained while the cost can be suppressed. Therefore, the Ca content when included is preferably 0.010% or less, and more preferably 0.004% or less.

[REM: 0 to 0.30%]

REM, like Ca, is an element that has the effect of miniaturizing inclusions in steel and improving toughness and ductility after quenching. In order to sufficiently develop this effect, the REM content is preferably 0.001% or more, and more preferably 0.002% or more. On the other hand, when the REM content is 0.30% or less, the effect can be sufficiently obtained while the cost can be suppressed. Therefore, the REM content when included is preferably 0.30% or less, and more preferably 0.20% or less.

Here, REM refers to a total of 17 elements including Sc, Y, and lanthanoid, and the content of REM means the total content of these elements. REM is added to molten steel using, for example, an Fe-Si-REM alloy, and this alloy includes, for example, Ce, La, Nd, and Pr.

[O: 0.0070% or less]

O is not an essential element and is contained as an impurity in steel, for example. O is an element that causes deterioration of the properties of the steel sheet, such as by forming oxides and becoming the starting point of destruction. Additionally, oxides present near the surface of the steel sheet may cause surface flaws and deteriorate the appearance quality. For this reason, the lower the O content, the better. In particular, since deterioration of properties can be suppressed by setting the O content to 0.0070% or less, the O content is preferably 0.0070% or less. The lower limit of the O content is not particularly limited and may be 0%, but the practical lower limit of the O content in the actual fishing industry and refining is 0.0005%.

[Sb: 0.100% or less]

The lower limit of the Sb content is not particularly limited and may be set to 0%. Sb is an element effective in improving the wettability and adhesion of plating. In order to obtain this effect, it is preferable to contain Sb in an amount of 0.001% or more. On the other hand, by setting the Sb content to 0.100% or less, defects occurring during manufacturing can be suppressed and a decrease in toughness can be suppressed. Therefore, it is preferable that the Sb content is 0.100% or less.

The remainder other than the above components is Fe and impurities. The base steel sheet may also contain impurities mixed in during the manufacturing process or the like. Examples of such impurities include Zn (zinc).

Above, an example of the chemical composition of the base steel sheet of the hot stamping steel sheet according to the present embodiment has been described in detail.

The area on which the surface treatment film of the steel sheet having the above chemical composition has been applied can be made into a hot stamped member having a tensile strength of about 1000 MPa or more by heating and quenching using the hot stamping method. Additionally, in the hot stamping method, press processing can be performed in a softened state at high temperature, so it can be easily molded.

The steel sheet for hot stamping according to the present embodiment does not require the use of a plurality of steel sheets, compared to the case of hot stamping after welding different types of steel sheets or the method of changing the cooling rate depending on the part by controlling the mold temperature. There is no need for pre-welding equipment or processes. Additionally, there is no need for equipment or running costs to change the temperature of the mold. Therefore, it is economically desirable.

<Manufacture of hot stamped members with parts of different strengths>

In skeletal parts for automobiles, the strength of some parts may be increased while the strength of other parts may be reduced for the purpose of absorbing energy during a collision. Parts having parts with different strengths in this way can be manufactured using a steel sheet for hot stamping in which a region in which a surface treatment film is applied to a part of the surface is formed as described above.

First, for example, a surface treatment film is applied to a part of the surface of a metal material such as a coiled steel plate, and areas with different emissivity are formed in advance. Then, the steel sheet for hot stamping according to this embodiment is obtained by performing various processing such as cutting or punching with a press. Additionally, the steel sheet for hot stamping according to the present embodiment can be obtained by applying a surface treatment film to a steel sheet that has been cut or punched by a press. Additionally, the emissivity can be continuously changed by changing the film thickness of the surface treatment film.

For example, a steel sheet for hot stamping to which a surface treatment film has been applied as described above is hot stamped. Examples of heating devices include electric heating furnaces, gas heating furnaces, far-infrared furnaces, and conventional heating devices equipped with infrared heaters. As shown in Figure 1, the temperature increase rate is fast in the area where the surface treatment film is applied (the dermal film area) because the emissivity is high and the heat transfer effect by radiation is large, but the temperature increase speed at the other area (the non-coated area) is slow. . The area of the oil film that is rapidly heated and reaches a high temperature is heated above the Ac3 point, where the metal structure transforms into the austenite phase. On the other hand, since the rate of temperature increase in the non-coated area is slow, even if the covered area reaches a temperature above the Ac3 point, the metal structure can be kept at a temperature below the Ac3 point without completely transforming into the austenite single phase. In this embodiment, the heating device to be used, etc., can be appropriately controlled so that the above-mentioned state is realized.

Next, the heated steel sheet is formed and cooled. Areas where the temperature of the steel material is raised above the Ac3 point, where the metal structure transforms into the austenite phase, are quenched and the strength increases, while areas where the temperature is heated below the Ac3 point and the transformation into the austenite single phase is not completed. The intensity is relatively lower. As a result, it is possible to obtain a part (i.e., a hot stamp member) having parts with different strengths, and further, the difference in strength between parts with different strengths can be measured by the Vickers hardness (load F) specified in JIS Z 2244 (2009). : 50kgf, 1kgf is about 9.8N), so it can be over 150Hv.

As described above, by changing the film thickness of the surface treatment film, it is possible to continuously change the hardness by utilizing the temperature increase rate to continuously change. In areas where the surface treatment film has a large film thickness, austenitization progresses the most because the temperature increase rate is high, and the strength increases due to the formation of martensite during quenching during cooling. On the other hand, in areas where the film thickness is small, the ratio of the austenite phase during heating is lowered, and the amount of martensite produced is reduced compared to the area where the film is thick, thereby lowering the strength. Additionally, in areas where a film is not applied, the austenite phase ratio during heating is lower, the amount of martensite produced is smaller, and the strength is lowered.

In this way, in the hot stamping steel sheet according to the present embodiment, by appropriately controlling the area to which the surface treatment film is applied and the film thickness, when manufacturing a hot stamping member using this hot stamping steel sheet, the strength can be different. Parts can be manufactured arbitrarily.

<Hot stamp absence>

The hot stamp member obtained as described above has a portion on the surface of the steel sheet that has a surface treatment film and a portion that does not have a surface treatment film. This surface treatment film contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and the adhesion amount of the _oxide Additionally, this surface treatment film has a silica content of 0 to 0.3 g/m2. The carbon black present in the surface treatment film of the hot stamping steel sheet used as the material for the hot stamping member is lost through the hot stamping process, and the metal oxide remains. The amount of _oxide attached to the surface treatment film of the hot stamping member

When measuring the Vickers hardness (load F: 50 kgf, 1 kgf is about 9.8 N) specified in JIS Z 2244 (2009), these hot stamped members have a portion showing the maximum hardness HVmax and a portion showing the minimum hardness HVmin. It is preferable that the hardness difference ΔHV between the maximum hardness HVmax and the minimum hardness HVmin is 150 or more.

Here, as is clear from the above description, both the portion showing the maximum hardness HVmax and the portion showing the minimum hardness HVmin exist in a region composed of a common material (i.e., the hot stamping steel sheet according to the present embodiment). do. Here, “common members” refers to the ratio of content of specific elements (e.g., C, Si, Mn, P, S, Al, Ti, B, N) in the 0.05 mm field of view near the center of the plate thickness ( This refers to a case where the ratio (ratio between members) is anywhere in the range of 0.80 to 1.2 times. For example, products that have the same composition when confirmed at the center of the plate thickness and are manufactured through the same manufacturing process are referred to here as “common members.” In addition, this common material may be composed of a single steel plate, or may be composed of a plurality of identical steel plates (i.e., steel plates for hot stamping according to the present embodiment) joined by any method.

Example

Hereinafter, embodiments of the present invention will be described, but the conditions in the examples are only examples of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is limited to these examples of conditions. That is not the case. The present invention can adopt various conditions as long as it achieves the purpose of the present invention without departing from the gist of the present invention.

As a base steel sheet, it is desirable to use a steel sheet with high mechanical strength (meaning various properties related to mechanical deformation and destruction such as tensile strength, yield point, elongation, drawability, hardness, impact value, fatigue strength, etc.). The chemical composition of the base steel sheet before plating used in the hot stamping steel sheets shown in the examples below is shown in Table 1 below.

A surface treatment film was applied to base steel sheets (steel Nos. S1 to S18) having the chemical components shown in Table 1. More specifically, as shown in Figure 2, in a steel plate with a width of 100 mm x a length of 200 mm and a thickness of 1.2 mm, a surface treatment film is applied to the upper half of 100 mm of the 200 mm in the longitudinal direction on one or both sides. This was used as a dermal film area, and the remaining 100 mm was left with no surface treatment film applied, making it a skin-free area.

In addition to the water-based acrylic resin that is the binder component, commercially available carbon black, TiO ₂ , ZrO ₂ , ZnO, Fe ₂ O ₃ , Fe ₃ O ₄ , CuO, SiO ₂ , TiC, TiN, SiC, SiN, etc. compounds can be used. At least one type of water-based treatment liquid added was applied to a portion of the base steel sheet using an industrial inkjet printer and dried to provide a surface treatment film. In addition, some aqueous treatment liquids contained silica in addition to the above components. The thickness of the surface treatment film was within the range of 1.0 to 2.5 μm, and when applied to both sides, the same type of film was applied to both sides.

After that, a thermocouple was connected to the center (P1) of the area where the surface treatment film was applied (enriched film area) and the center (P2) of the area without the film (non-film area) shown in FIG. 2, respectively, and the temperature at each position was measured. made it possible to measure. Then, the steel sheet was heated in an electric heating furnace at a set temperature of 900°C, and when the area where the film was applied reached 880°C, the steel sheet was taken out from the heating furnace. The steel plate was rapidly cooled using a flat mold to obtain a hot stamp member. In addition, the area to which the surface treatment film was applied (the dermal film area) becomes the area showing the maximum hardness HVmax when the Vickers hardness of the manufactured hot stamp member is measured, and the area to which the surface treatment film is not applied (the non-coated area) is the area showing the maximum hardness HVmax. When the Vickers hardness of the manufactured hot stamp member is measured, the area shows the minimum hardness HVmin.

Using a radiation thermometer, the radiation intensity at the center P1 of the area where the surface treatment film was applied was measured at a wavelength of 8.0 μm at 25°C, and the emissivity (%) was calculated from the ratio to the radiation intensity of the black body.

In addition, some base steel sheets are subjected to Al-10% by mass Si plating, Zn plating, or Al plating and then given the above surface treatment film, or by electroplating with 3% by mass of Zn-plating. After Ni plating was performed, the above surface treatment film was applied. In the case of the hot dip plating method, the base steel sheet was immersed in the plating bath, and then the adhesion amount was adjusted to 70 g/m2 per side using the gas wiping method. In the case of electroplating, the single-side adhesion amount was adjusted to 20 g/m2.

The film composition of the area where the surface treatment film was applied and the difference in Vickers hardness between P1 and P2 of the obtained hot stamp member were investigated, and in some examples, appearance, paint adhesion, and corrosion resistance after painting were also investigated. .

The evaluation method for each evaluation item was as follows.

(1) Strength characteristics

(grade)

Using the method described in Japanese Industrial Standard JIS Z 2244 (2009), the Vickers hardness at the center P1 of the area where the surface treatment film was applied and the center P2 of the area where the surface treatment film was not applied were measured from the cross section of the steel sheet ( Load F: 50kgf). The strength characteristics were evaluated based on the difference in hardness between P1 and P2, and when the difference in hardness was Hv150 or more, the member was considered to have different strengths and excellent strength characteristics.

3: Vickers hardness difference ΔHV is Hv200 or more

2: Vickers hardness difference ΔHV is Hv150 or more and less than 200

1: Vickers hardness difference ΔHV is less than Hv150

(2) Appearance

(grade)

Measure the CIE 1976 L*a*b* color space at 5 locations using the method described in Japanese Industrial Standard JIS Z 8781-4 (2013), and compare the ratio of L* values when comparing 2 arbitrary locations (RL*=L *Value 1/L*Value 2) was evaluated.

2: 0.5 to 2.0

1: less than 0.5 or greater than 2.0

(3) Paint adhesion

The sample was subjected to phosphoric acid chemical conversion treatment and electrodeposition coating with a thickness of 15 μm, and baked at 170°C for 20 minutes to provide a coating film. After that, after immersing in deionized water at 60°C for 200 hours, 100 checkerboard patterns at 1 mm intervals were formed with a cutter, and the number of peeled parts of the checkerboard parts was visually measured to calculate the area ratio of the peeled parts. did. Based on the calculated area ratio, a rating was given.

(grade)

3: Peeling area 0% or more but less than 10%

2: Peeling area 10% or more but less than 70%

1: Peeling area 70% or more and 100% or less

(4) Corrosion resistance after painting

As in (3), the applied coating film was scratched with a cutter, and this was done by the method specified in JASO M609 established by the Society of Automotive Engineers. The width (maximum value on one side) of the coating film expansion from the cut flaw after 180 cycles of the corrosion test was measured.

(grade)

3: Expansion width 0 mm or more but less than 1.5 mm

2: Expansion width 1.5 mm or more but less than 3 mm

1: Expansion width of 3 mm or more

The evaluation results obtained in Examples 1 to 4 conducted below are shown in Tables 2, 3, 4, and 5, respectively.

<Example 1>

In Table 2 shown below, A1 to 21 are examples, and a1 to a3 are comparative examples.

In this Example 1, when adjusting the aqueous treatment liquid, at least one of carbon black, titanium nitride, titanium carbide, titanium oxide, iron oxide, copper oxide, zirconium oxide, silicon nitride, cobalt oxide, and tin oxide is used as a compound other than the binder component. was used. The total content of compounds other than the binder component in the aqueous treatment liquid was set to be within the range of 2 to 50% by volume based on the volume of the entire solid content. The solid concentration of the aqueous treatment liquid was set to 10 to 40% by mass, and the liquid was applied on a steel plate to a film thickness of 3 μm to 25 μm and then dried to obtain a film. The atmosphere during drying was air or nitrogen, and the temperature was 100 to 300°C. The total content of compounds in the aqueous treatment liquid becomes the total content of compounds in the surface treatment film obtained after drying. In this example, the emissivity value at a wavelength of 8.0 μm was adjusted by adjusting the compound content and the adhesion amount of the surface treatment film.

In Comparative Examples a1 to a3, the emissivity at 25°C and a wavelength of 8.0 μm in the dermal film region P1 was small at 58, 56, and 58%, and the hardness difference between the dermal film region P1 and the non-film region P2 of the hot stamp member was , it was less than ΔHV150 (rating 1) on Vickers hardness. Since the emissivity of the films a1 to a3 at a wavelength of 8.0 μm at 25°C is less than 60%, there is no significant difference in the temperature increase rate during heating between the covered area P1 and the non-covered area P2, and hot stamping It is estimated that the Vickers hardness difference of ΔHV150 or more did not occur even in the structure of the member.

On the other hand, in invention examples A1 to A21, the emissivity at 25°C and a wavelength of 8.0 μm was 60% or more. As a result, the hardness difference between the dermis filmed portion and the non-coated portion of the hot stamp member was HV150 to HV200 (rating 2) in terms of Vickers hardness.

<Example 2>

In Table 3, in invention example B1, carbon black (CB) was set to 2.7 volume% in the surface-treated film, and TiO ₂ was set to 0.6 volume% in the surface-treated film. As a result, the emissivity at 25°C and a wavelength of 8.0 ㎛ in the dermis filmed area is 86%, and the hardness difference between the dermis filmed part and the uncoated part of the hot stamp member is Vickers hardness of HV150 to less than 200 (score 2). It has been done. The paint adhesion was rated 3, but the appearance was rated 1. In addition, in invention example B2 in which carbon black was 4.0% by volume in the surface-treated film and TiO ₂ was 1.0% by volume in the surface-treated film, the emissivity at 25°C and a wavelength of 8.0 μm in the oil film portion was 80%. , the hardness difference between the oil-coated portion and the non-coated portion of the hot stamp member became Vickers hardness HV150 to HV200 (rating 2). Invention Example B5 in which carbon black was 58.3 volume% in the surface-treated film and ZnO was 1.2 volume% in the surface-treated film, and carbon black was 47.3 volume% in the surface-treated film and ZnO was 1.0 volume% in the surface-treated film. In invention example B6 in %, the emissivity at 25°C and a wavelength of 8.0 μm in the dermal film portion was 88% and 90%, respectively, and the appearance was rated 2. Additionally, the paint adhesion was rated 2.

On the other hand, in invention examples B3 to B4 in which carbon black was 5.0% to 40.0% by volume in the surface treatment film, the emissivity at 25°C and a wavelength of 8.0 μm in the oil film portion was 82% and 86%, The hardness difference between the oil-coated portion and the non-coated portion of the hot stamp member was Vickers hardness HV200 or more (rating 3), and the appearance was rated 2 and paint adhesion was rated 3.

From the above results, as in invention examples B3 to B4, molded articles were obtained that were excellent in not only strength characteristics but also appearance and paint adhesion by including 5.0 to 40.0 volume% of carbon black.

<Example 3>

In Table 4, Invention Example C1 in which the content of Zn oxide in the surface-treated film was set to 0.2 volume%, Invention Example C2 in which the content of Ti oxide in the surface-treated film was set to 0.3 volume%, and Ti oxide in the surface-treated film. Compared to invention example C6 in which the content was 37.5 volume%, as in invention examples C3 to C5, the content of Ti oxide and Zr oxide in the surface treatment film was set to 1.0 to 30.0 volume%, so that there was no contact with the oil film portion of the hot stamp member. The hardness difference in the film area became larger (rating 3). The reason for the increased hardness difference is that the emissivity of the oxide is greater than that of the compound on the surface of the steel sheet, and the reflection of infrared rays is inhibited by the presence of the interface between the surface of the steel sheet and the oxide particles, thereby increasing the amount of heat input to the sample, which increases the amount of heat input to the sample. It is thought that the increased difference in heating speed is a factor.

<Example 4>

In Table 5, in invention example D1, the emissivity at 700°C and a wavelength of 8.0 ㎛ in the dermal film portion is 56%, and the hardness difference between the dermal film portion and the uncoated portion of the hot stamp member is Vickers hardness of HV150 or more and 200. It became less than (rating 2). On the other hand, in invention examples D2 to D6 in which the emissivity at 700°C and a wavelength of 8.0 μm in the dermis film portion was 60% or more, the hardness difference between the dermis film portion and the non-coat portion of the hot stamp member was Vickers hardness HV200 or more (rating 3). It became.

<Example 5>

In Table 6, as is clear when comparing invention examples E1 to E5 and E6, in invention example E6 without plating, the corrosion resistance after painting was "1", while Al-10% by mass Si, or Zn-3% by mass. In invention examples E1 to E5 having % Ni plating, the corrosion resistance after coating was improved to “2” or “3”.

As described above, according to the present invention, by applying a surface treatment film to a desired part of a steel sheet and hot stamping it, it was possible to form parts of different strengths in the obtained hot stamped member.

Although preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that those skilled in the art in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the patent claims, and of course, these are also possible. It is understood to fall within the technical scope of the present invention.

According to the present invention, it becomes possible to rapidly heat an area provided with a surface treatment film having an increased emissivity at a wavelength of 8.0 μm at 25°C by increasing heat transfer by radiation. By providing differences in hardness, it is possible to prepare regions of different strengths among hot stamped members from a single steel sheet. Therefore, the possibility of industrial use is high.

Claims (9)

The surface of the steel sheet has a portion having a surface-treated film having an emissivity of 60% or more at a wavelength of 8.0 μm at 25° C. and a portion not having the surface-treated film,
The surface treatment film contains carbon black and at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide, and the carbon black and the oxide are the entirety of the surface treatment film. It exists dispersedly in
The surface treatment film has a silica content of 0 to 0.3 g/m2,
A steel sheet for hot stamping that satisfies the following equation (1) when the adhesion amounts of the carbon black and the oxide are respectively X _CB (g/m2) and X _Oxide (g/m2).
118.9≤24280/{6700/(100+76×X _CB )+18000/(130+65×X _Oxide )}≤332.0 … Equation (1) According to paragraph 1,
A steel sheet for hot stamping, wherein the surface treatment film contains 5.0 to 40.0% by volume of the carbon black and 1.0 to 30.0% by volume of the oxide. According to claim 1 or 2,
A steel sheet for hot _{stamping ,} wherein the ratio of the adhesion _amount of carbon _black According to any one of claims 1 to 3,
A _steel sheet for hot stamping, wherein the carbon _black adhesion amount According to any one of claims 1 to 4,
A steel sheet for hot stamping, wherein the surface treatment film has an emissivity of 60% or more at a wavelength of 8.0 μm at 700°C. According to any one of claims 1 to 5,
A steel sheet for hot stamping, which has a metal plating layer on one side or both sides of the steel sheet for hot stamping, between the base material of the steel sheet and the surface treatment film. On the surface of the steel sheet, there is a portion having a surface treatment film and a portion not having the surface treatment film,
The surface treatment film contains at least one oxide selected from the group consisting of Zr oxide, Zn oxide, and Ti _oxide , and the adhesion amount
A hot stamp member wherein the surface treatment film has a silica content of 0 to 0.3 g/m2. In clause 7,
When measuring the Vickers hardness specified in JIS Z 2244 (2009), there is a part showing the maximum hardness HVmax and a part showing the minimum hardness HVmin, and the hardness difference ΔHV between the maximum hardness HVmax and the minimum hardness HVmin is , over 150, no hot stamps. According to clause 8,
A hot stamp member wherein both the portion showing the maximum hardness HVmax and the portion showing the minimum hardness HVmin exist in a region composed of a common material.

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